HNO plays significant roles in many biological processes, such as vascular relaxation, enzyme activity regulation, and neurological function regulation. It also offers a promising new treatment for diseases such as heart failure and stroke. Many HNO biological effects occur with metalloproteins. However, the atomic level structural and functional information are largely unknown. Our long-term goal is to determine how HNO binds with its biological targets and how such binding leads to protein functional responses. Aim 1 is to determine the atomic level HNO binding modes in myoglobin (Mb). Heme proteins are frequently involved in HNO binding. But there are no X-ray structures of any HNO protein complexes. The first stable HNO complex is with Mb, enabling numerous spectroscopic characterizations, which were used by us recently with computational work to propose two novel active site models. These models exhibit unique novel features, which need to be validated with protein level investigations. Our preliminary protein level studies has shown support of our active site results, but also offered more details of effects on nearby groups. We will perform protein level investigations of various binding modes and compare with more experimental results not investigated before, to provide the first rigorous atomic level HNO binding picture with Mb. Results will facilitate investigations of HNO binding with other heme proteins. Aim 2 is to determine HNO/NO conversion mechanism via Cu,Zn-SOD and a Cu complex with imaging function. Reaction of HNO with Cu,Zn-SOD was suggested to be important for in vivo formation of NO. A copper complex was recently reported to have similar reaction and allow selective detection and imaging for HNO. Our preliminary studies found interesting reaction pathways for them. We will study reactions using different scales of active site models and protein environment to help understand their effects on mechanisms. Additional mechanistic investigations of the Cu complex with modified ligands will be done to provide design guidelines for HNO imaging agents. Aim 3 is to determine HNO/NO conversion mechanisms via Mb. Our preliminary studies showed some interesting reaction mechanisms to support experimental results. We will perform a series of computational investigations including protein level calculations to validate the mechanisms. As heme models were recently used to trap HNO, we will also systematically evaluate effects of metal centers and heme ligands to help future design of HNO scavengers. Results will provide useful structural and mechanistic results of HNO interactions with metalloproteins and models to facilitate studies of health, diseases, and therapeutic treatments involving HNO.